A conventional chip bin serves primarily to ensure a steady flow of wood chips to the subsequent steaming and digesting stages in pulp production. Chips are brought in from a wood chip yard by a conveyor, and fed into the top of the chip bin. The chips are then transferred out of the bottom of the chip bin using a feeder valve or chip meter, which produces a constant flow of chips, into a steaming vessel. In the steaming vessel, the chips are heated with steam at superatmospheric pressure, so that non-condensable gases are removed from the chips and the chips become impregnated with liquid, primarily condensed steam. The chips are then discharged from the steaming vessel into a chip chute where liquor is introduced, then via a high-pressure feeder into a digester where they are converted into pulp.
U.S. Pat. No. 5,500,083 to Johanson describes a chip bin in which low-pressure steam is introduced into the chip bin to start the steaming process before the chips are transferred to a conventional steaming vessel.
U.S. Pat. No. 5,736,006 (Prough) describes a process using Johanson's chip bin. Prough proposes heating the chips in the chip bin to a temperature of 110° C. or less, preferably to 105° C. or less in steam at 3 psig (20 kPa) or less, and suggests that it is then possible to dispense with a separate steaming stage.
Neither Johanson nor Prough describes the steaming process in detail, and the present applicant has found that the system implied by Johanson's construction, with steam introduced at low pressure through inlets in the outer wall of the chip bin, is not very effective.